The value of the composite piezoelectric material is characterized by its ability to capture signals emitted by the substrate material to which it is attached. The signals may e.g. originate from vibrations, crack formation, torsion, movement or transformations of the substrate material.
The document U.S. Pat. No. 5,702,629 discloses methods and compositions for preparing piezoelectric ceramic-polymer composites in the form of compacted disks.
More recently composite piezoelectric materials have been described in the form of pastes or paints, i.e. in the form of a mixture of a soft and malleable consistency. These materials are also referred to as piezoelectric thick film materials or piezoelectric paint. In particular the usability of such materials for structural health monitoring applications has attracted considerable attention,
For example, X. Li and Y. Zhang: “Analytical study of piezoelectric paint sensor for acoustic emission-based fracture monitoring”, in Fatigue & Fracture of Engineering Materials & Structures, Volume 31, Issue 8, pages 684-694, August 2008, have studied the properties of a piezoelectric paint for the acoustic emission technique for non-destructive evaluation such as fracture monitoring in metal, concrete and composite structures. Composites according to this reference comprises piezoelectric paints constituted of tiny piezoelectric particles mixed within polymer matrix and belonging to the ‘0-3’ piezoelectric composite meaning that the piezoelectrically active ceramic particles are randomly dispersed in a three-dimensionally connected polymer matrix. The advantages of this material is its ease of fabrication into complex shapes including large flexible thin sheets, extruded bars and fibres and moulded shapes and the material may conform to any curved surface.
V. Giurgiutiu and Bin Lin, “In-situ Fabricated Smart Material Active sensors for structural Health Monitoring”, in Smart Materials III, ed. By Alan R. Wilson, Proceedings of SPIE Vol. 5648, have identified in-situ composite piezoelectric wafer active sensors as good candidates for reliable low-cost options for Structural Health Monitoring (SHM) smart sensor fabrication due to the ease with which their mechanical properties may be tailored, low cost ease of implementation, conformability to curved surface and compatibility with polymeric materials. The piezoelectric composite is made of a PZT powder added to an epoxy matrix phase, the paste is then spread into a mask and allowed to cure at an elevated temperature around 50° until hard. At last excess is removed and the composite is sanded down to final thickness.
I. Payo and J. M. Hale, “Dynamic characterization of piezoelectric paint sensors under biaxial strain”, in Sensors and Actuators A 163, 2010, page 150-158, discloses a piezoelectric paint film designed to be used as a strain sensor. The composite is a suspension If milled PZT (lead zirconate titanate) ceramic powder in a polymer binder in the form of a water-based acrylic. The piezoelectric material film is applied to the substrate with a conventional spray gun directly to the top surface of the substrate. It is pointed out in the paper that the described piezoelectric paint sensors present some advantages with respect to other film sensors such as e.g. PZT ceramics and polyvinylidene fluoride (PVDF).
R. Lahtinen et al., “A piezopaint-based sensor for monitoring structure dynamics”, in Smart Materials and Structures, 16, 2007, page 2571-2576, disclose a combination of a piezoelectric powder with a paint resin such as an epoxy resin which resulting paint can easily be applied to almost any surface. The paint is used as a vibration sensor e.g. on a footbridge and the paint is applied with a laboratory application such as a doctor blade with a fixed gap or alternatively by spraying. In the text page 2571, col. 1, lines 10-14, it is mentioned that vibration and strain monitoring commonly is solved by using piezoelectric polymers such as Polyvinylidene fluoride (PVDF) or strain gauges but the thermal stability of PVDF is limited to 80° and the strain gauges require elaborate temperature compensation.
K. Arlt and M. Wegener, “Piezoelectric PZTIPVDF—copolymer 0-3 Composites: Aspects on Film Preparation and Electrical Poling”, Fraunhofer Institute for Applied Polymer Research (IAP), IEEE Transactions on Dielectrics and Electrical Insulation Vol. 17, No. 4, August 2010, discloses composite films of PZT (lead zirconate titanate) and different i.e. non-polar and polar PVDF (Polyvinylidene fluoride) copolymers prepared as 30-150 μm thick freestanding flexible films. The article considers the influence of the poling time and the poling-temperature regimes on piezoelectric properties achieved in the composite. The composite films are prepared by solvent casting the mixtures onto glass plates
The document U.S. Pat. No. 5,951,908 disclose compositions and processes for fabrication of piezoelectric composites comprising piezoelectric particles embedded in a polymer matrix having improved figures of merit for both sensor and non-sensor applications. The improvements result from discoveries of the effects of polymer bulk compliance, polymer anisotropy, polymer melt index and polymer/ceramic wettability on performance. In order to avoid substantial void volume in the composite, the polymer matrix should wet the ceramic particles properly and diffuse into the gaps between the ceramic particles to form a cavity-free composite. Use of curable resins such as epoxy and polyurethane resins can be used to make 0-3 composites according to this document.
The document U.S. Pat. No. 4,917,810 discloses a piezoelectric composite material prepared by compounding a ferroelectric ceramic powder comprising microcrystals having virtually single domains and a polymer. The ferroelectric ceramic powder can be mixed with a wide variety of polymers, molded in an arbitrary shape e.g. a sheet, subjected to a hardening treatment such as cross-linking or vulcanization and then polarized to prepare the piezoelectric composite material.
Definitions
PZT—lead (P) zirconate (Z) titanate (T) (piezoelectric ceramic)
KNN—Alkali Niobate based material e.g. K0.5Na0.5NbO3 
AE—Acoustic Emission
d33—piezoelectric charge constant (p C/N=pica (10−12) Coulomb/Newton)
It should be emphasized that the term “comprises/comprising” when used in this specification is taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.
“Satisfying wetting properties” indicates that a contact angle of less than 90° is obtained between fluid and solid substrate.
“Substrate” is the technical name used to address any parts or materials to which a film or layer of composite piezoelectric material is attached and is intended to finally function, it is critical to match substrates and piezoelectric paste as to their chemical compatibility such as e.g. wettability. Substrates need to be clean and free from surface contamination to allow proper paste adhesion.